Aircraft turboprop engine inlet compact profile configuration

ABSTRACT

A turboprop powered aircraft having a cowling or nacelle surrounding the engine, said cowling incorporates an air inlet and internal ducting to the engine wherein the inlet incorporated anti-icing ducts, said inlet shape has a closed oval or other non-circular shape that is not coaxial to the engine propeller drive shaft. Inlet anti-icing heating duct means are provided for channeling a portion of the engine&#39;s exhaust to the interior of the inlet rim. A further improvement in inlet efficiency can be achieved by reducing the thickness of the inlet cross-section, but this results in less space available for the hot exhaust connections. To provide for the inlet reduced section thickness, the exhaust flow connections also incorporate reduced thickness sections to facilitate cooling and insulation of the adjacent cowling and inlet duct surfaces from the excessive heat associated with the hot exhaust gas connection. Additional cooling means utilize air flow from openings in the engine cowling that are channeled to cool internal surfaces of the cowling that may be exposed to excessive heat.

This application claims the benefit of priority pursuant to 35 U.S.C. 119(e) from a U.S. Provisional patent application having Application No. 62/923,968 filed Oct. 21, 2019, the text of which is fully incorporated by reference herein, as if repeated, below.

BACKGROUND OF THE INVENTION

The design of the engine intake air inlet for a turboprop powered aircraft has a significant effect on the power output of the engine, particularly at higher altitudes. This is because a large portion of the power generated by a turboprop engine is consumed by the engine's compressor section. Improvements in the design of the air inlet and associated ducting that increase the ram-air recovery to the engine intake, can result in higher compressor efficiency.

A conventional, less efficient inlet design of prior art is shown in FIG. 1. In this design the upper surface of the air inlet is an extension of the cowling surface and is approximately in-line with the propeller spinner's outer surface. It was thought that this approach would have minimal drag, but the turbulent air surrounding the spinner greatly reduces the pressure recovery characteristic of the inlet.

Another prior art approach has been to use a flush inlet similar to the designs outlined in NACA Report ACR No. 5120 The flush inlet designs have an advantage that they generally do not require separate means to deice the inlet, but the turbulent air surrounding the spinner also greatly reduces the pressure recovery characteristic of the inlet.

A significant improvement in the inlet ram-air pressure recovery associated with prior art designs is shown in FIG. 3A. In this design the inlet opening has been moved so that all or most of the turbulent air exiting the spinner does not enter the inlet. The offset opening location generally incorporates a cavity recess region in the cowling aft of the spinner and above the inlet that provides an alternate flow path for the turbulent boundary layer air exiting the spinner. The design shown in FIG. 3A is commonly referred to as a Pitot Cowling, or Ram-Air Cowling.

Prior art has recognized the importance of incorporating means to prevent ice from forming on the inlet surfaces. It is also critical to insure adequate air flow to the engine is maintained when the aircraft is operated in icing conditions. The means to prevent ice from forming or removing ice once formed, are typically referred to as anti-ice or de-ice systems. These systems generally involve heating the inlet surfaces. Sources of heat include siphoning off a portion of pressurization bleed air or engine exhaust. Electrical heating elements have been used extensively. Alternatively, some systems employ ice melting fluids that are continuously applied to the outer surfaces of the inlet when the aircraft is operated in icing conditions.

SUMMARY OF THE INVENTION

Although the basic design of a typical Ram-Air Cowling is well understood, this invention adds several unique design details that when incorporated individually or in combination result in an improvement in the operation of the inlet. For improved ram air recovery to the engine, the inlet cross-sectional thickness is reduced. Where the inlet anti-icing means are provided by extracting a portion of the engine's exhaust gases or compressor bleed air, additional means are provided to cool the cowling and inlet duct surfaces that may be exposed to excessive heat associated with the inlet exhaust gas connection means. To accomplish this, external air flow from separate openings in the engine cowling is channeled through ducting to specific regions of the internal surfaces of the cowling and the external surfaces of the engine air intake ducting that may be exposed to heat from the exhaust gases entering or exiting the inlet. This surface cooling is particularly important for cowlings and ducting made of composite materials, such as epoxy pre-impregnated carbon fibers.

In current practice, round tubes are connected directly to the inlet to channel the hot gasses to and from the inlet lip. However, using thinner inlet sections of the preferred lip design described herein results in less space for the exhaust flow connections. Consequently, when using thinner inlet sections of the preferred lip design with round tubes directly connected to the inlet lip, the diameter of the round tubes would need to be significantly reduced, and the round tubes may not have sufficient cross-sectional area to insure adequate hot gas flow without interfering with the cowling and inlet duct surfaces. To provide the needed flow area according to the invention, connection adaptors are used that transition from a round tube section to a non round section at the attachment location on the inlet wherein sufficient flow area is maintained.

The invention transition connection profile is designed to reduce heating of the adjacent internal surfaces of the cowling as well as the external surfaces of the engine airflow inlet duct. The design of the transition connection invention also provides a channel for external air to be ducted to cool the cowling and duct surfaces. The invention also describes means to channel air flow to cool surfaces near the transition connection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the prior art, non-ram-air design inlet used on the Pilatus PC-12 and 21 model single engine turboprop as well as other aircraft.

FIG. 2 is a perspective view of a ram-air inlet design suitable for turboprop powered aircraft including the Pilatus PC-12 and 21 models, incorporating the design aspects of the invention.

FIG. 3A is a vertical plane view through the engine centerline showing atypical cross-section of the prior art ram-air inlet designs currently used on turboprop powered aircraft.

FIG. 3B is a vertical plane view through the engine centerline showing a typical cross-section of the reduced section thickness ram-air inlet design according to the invention.

FIG. 3C is a vertical plane view showing an overlay comparison of the sections shown in FIGS. 3A and 3B.

FIG. 4 is a perspective view of the prior art round tube exhaust heating connections to the air inlet typical of designs currently used on turboprop powered aircraft.

FIG. 5 is a side cross-sectional perspective view of a cut-away of the cowling in FIG. 2 showing the engine exhaust heating connections and components for the ram-air inlet design according to the invention.

FIG. 6 is a side cross-sectional perspective view of a cut-away of the cowling in FIG. 2 showing details of the connections and components for the ram-air inlet design according to the invention.

FIG. 7 is an aft perspective view of the circular to flattened or oval, transition connection to the inlet for this invention.

FIG. 8 is an aft perspective view of the circular to flattened, or ovoid, transition connection to the inlet lip, or rim, described for this invention, with an attached heat shield.

FIG. 9A is a front view of the circular to flattened, or ovoid, transition connection described in this invention.

FIG. 9B is a top view of the circular to flattened, or ovoid, transition connection described in this invention.

FIG. 9C is an aft view of the circular to flattened, or ovoid, transition connection described in this invention.

FIG. 9D is a side view of the circular to flattened, or ovoid, transition connection described in this invention.

FIG. 10 is an aft view of the internal region of the cowling in FIG. 2 showing the circular to flattened, or ovoid, transition connection region with cooling air ducting.

FIG. 11A is a front view of the cowling in FIG. 2 showing some of the dimension references and the NACA inlets supplying air to the internal cooling air ducting shown in FIG. 10.

FIG. 11B is vertical plane view through the engine centerline showing atypical cross-section, taken along lines C1-C2 of FIG. 11a , of the reduced section thickness ram-air inlet design according to the invention.

FIG. 12 is a side cross-sectional perspective view of a cut-away of the cowling in FIG. 2, taken along a centerline of the cowling, showing the addition of insulation materials and an alternate transition connection region cooling air design using a plenum chamber.

FIG. 13 is an aft view of the alternate transition connection region with cooling air design shown in FIG. 12.

FIG. 14A is an exploded aft perspective view of the exhaust tube connections and support of FIG. 13.

FIG. 14B is an aft perspective view of the exhaust tube connections and support of FIG. 14A assembled.

PRIOR ART DISCUSSION AND COMPARISON

The two basic prior art cowling configurations used on aircraft powered by turboprop engines are shown in FIGS. 1 and 3A. Referring to FIG. 1 for the first configuration, an aerodynamically contoured nacelle or cowling 1 surrounds the engine compartment with openings to accommodate the engine's exhaust stacks 14 a. An inlet 2 providing intake air to the engine is positioned behind the propeller 24. In most installations, the inlet 2 is not co-axial to the propeller centerline but is oriented either above or below the propeller spinner 23, with below generally being preferred. For the configuration shown in FIG. 1, the upper surface of the inlet air duct 25 to the engine is approximately tangent to the aft surface of the spinner, with the design goal of having the air flow uniformly and with minimal flow loss from the spinner into the inlet. As anti-icing provisions for the lip of the inlet are generally required by the operational specifications of most aircraft, the anti-icing provisions need only be applied to the lower, crescent portion of the inlet lip, or rim.

The configuration shown in FIG. 3A represents what is commonly referred to as a ram-air cowling design. The ram-air cowling design is generally preferred over the configuration shown in FIG. 1 because the position of the inlet results in higher pressure recovery to the intake of the engine. Referring to FIG. 2, an aerodynamically contoured nacelle or cowling 3 having external surface 45, surrounds the engine compartment with openings to accommodate the engine's exhaust stacks 14 a. An inlet 5 providing intake air to the engine is positioned just behind propeller 24 and generally below the propeller spinner 23. For the ram-air configuration the entrance to the inlet 5 is positioned a sufficient distance below or above the surface of the spinner 23 to provide a gap region 26 for air flow exiting the spinner 23 to not enter the inlet 5. This is because the airflow near the spinner 23 is more turbulent and has less potential dynamic pressure than air flow further out from the centerline of the propeller 24. To capture the higher velocity air exiting the propeller, the inlet is position close to the trailing edge of the propeller blades. Current FAA regulations require at least 0.5-inch clearance between the inlet lip and propeller blade trailing edges when the propeller blades are in the feather positions.

As anti-icing provisions for the lip of the inlet 5 are generally required by the operational specifications of most aircraft, the anti-icing provisions need to be applied to the entire lip region of the inlet 5.

FIG. 3A illustrates the internal arrangement of a typical ram-air cowling inlet design in the prior art. The engine's intake air enters the inlet 5 and flows through the internal passage 6 of duct 8 to the engine's intake plenum chamber. A common means to deice the inlet 5 is to siphon off a portion of the hot exhaust exiting the exhaust stack 14 a, and channel the hot gas flow through the inlet 5. For this, round tubing 15 a is used to connect the inlet 5 to the exhaust stack 14 a. The diameter (reference E in FIGS. 3A and 3B) of the tube 15 a must be large enough to insure the area of the tubing is of sufficient size to achieve an adequate flow rate through the interior channel 7 of the inlet 5 to fully deice the inlet 5 surfaces. In prior art designs this requires that the inlet cross-sectional thickness (reference D in FIG. 3C) in the region of the exhaust tube connection be larger than the diameter of the connection tube 15 a. In prior art typical designs, the inlet 5 cross-sectional thickness D in the region of the connection will be over 2 times the tube diameter E to provide sufficient clearance to the adjacent surface of the cowling 3 and duct 8.

DETAILED DESCRIPTION

FIG. 3B illustrates the preferred internal connection arrangement provided in the present invention. The inlet centerline upper section leading edge point 33 is positioned at distance S from the engine centerline. The distance S is generally about 1.2 to 1.5 times the maximum radius, R, of the spinner, and preferably between 1.27 to 1.33 times the maximum radius of the spinner. The inlet centerline upper section leading edge point 33 is also positioned aft the spinner aft bulkhead (spinner & cowl junction) by distance T. As mentions the distance T must be sufficient to provide the FAA require minimum clearance with the propeller blades. For most installations the distance T is about between 0.5 and 1.1 inches but may be more if the blades have a larger aft sweep.

Referencing FIG. 3C, Item 45 refers to the outer surface of a prior art cowling. Item 44 refers to the outer surface of the cowling incorporating the invention concepts discussed herein, and Item 3 a refers to the inner surface of the said cowling. Item 8 refers to the internal duct channeling the airflow to the engine, said duct having and external surface 8 a, and internal surface 8 b.

A contribution to an improvement in inlet efficiency was achieved by reducing the thickness of the inlet lip 4 cross-section (reference C in FIGS. 3C, 11A, and 11B). Referencing a vertical plane through the engine drive shaft centerline 28, in the preferred section geometry, the sections in or parallel to plane 28 along the top of the inlet are not symmetric about the horizontal line (alternatively a line 31 perpendicular to the inlet face plane and through the center point 33 on the leading edge radius forward-most line 29, separating the inlet section upper profile 35 a and lower profile 35 b see FIGS. 11A and 11B). In the preferred application the reduced sections covers a lateral distance J1 and J2 from each side of the vertical plane 28 through the engine shaft centerline of approximately 75% of the inlet half widths K1, K2 measured from the center vertical plane 28 to the outer most point 30 a or 30 b on the inner wall surface of the inlet leading edge curve 29. When measured at the inlet throat (the location of intake-air flow minimum cross-sectional area) location B aft of the section leading edge 29, the perpendicular distance from the horizontal line used to define the section thickness of the lower portion (C1) is greater than the thickness of the upper portion (C2) where C=C1+C2. The preferred ratio C2/C is between 0.25 and 0.40, and the most preferred ratio is between 0.30 and 0.35.

For a given lip offset distance S (FIG. 3 B,C) from the engine, or spinner, centerline, and gap 26 region minimum surface fillet radius, Z, the reduced upper lip section thickness C allows the gap region, 26, to extend further aft, reference distance X for prior art (FIG. 3C), and distance Y for this invention shown in FIG. 3C. This allows more space for the low dynamic pressure, turbulent air flow exiting the spinner region to not converge with the air flow entering the inlet 4. The reference distance Y from the propeller spinner aft bulkhead reference plane for this invention is typically 20 to 30 percent longer that the distance X for prior art.

This reduction in thickness (D-C) of the ram-air inlet lip 7, results in less space available between the duct 8 outer surface 8 a and internal surfaces 3 a of the cowling 3, potentially resulting in excessive heating of those surfaces from their close proximity to the exhaust flow heated connection. To compensate for the reduced thickness of the inlet lip 4, exhaust gas flow transition connections 9 a and 9 b (FIG. 5) incorporating reduced thickness (reference F in FIG. 3B) connections are used; these allow for the provision for the cooling of, and insulation for, the adjacent cowling internal surface 3 a and ram-air inlet duct 8 external surfaces 8 b from the excessive heat associated with the hot exhaust gas connection. The transition connections 9 a or 9 b can be braised, welded or otherwise permanently attached to the inlet 4 partition bulkhead 4 a or may be removably attached such as the bolted flange 9 c shown in FIG. 9. Typically, to provide sufficient clearance, the height F of the transition connections 9 a and 9 b should be under 50% and preferably around 40% of the inlet 4 section thickness C (F/C=0.4). To maintain the necessary exhaust flow area, the lateral direction width of the transition connections 9 a and 9 b (reference G and G′ FIG. 9B) are increased proportionally.

To achieve improved inlet duct ram-air pressure recovery, the inlet section has a reduced thickness C in the region of the exhaust flow connection of between 25% and 35% and preferably between 26% and 28% of the height A of the inlet 4 opening (FIG. 3C) measured at the inlet throat, i.e., at the location a distance “B” in the duct from the forwardmost end of the duct lip 33. Alternatively, the inlet section reduced thickness C can be defined as a fraction of the distance B from the inlet face plane to the inlet throat wherein the preferred fraction ranges from 55% to 75% of the distance B with a preferred range of 60% to 70%.

As an example applying the invention to a single engine aircraft, shown in FIG. 2, such as the Pilatus PC-12 or 21 model aircraft, using the first method, and referring to FIG. 3C, the inlet duct throat diameter, A, is approximately 6 inches and the inlet section reduced thickness C in the region of the exhaust flow transition connection is approximately 1.63 inches for a C/A ratio of 0.27. Alternatively, the inlet section reduced thickness C can be defined as a fraction of the distance B from the inlet face plane to the inlet throat, wherein the preferred fraction ranges from 60% to 75% of the distance B with a preferred range of 60% to 70%.

As an example of the invention applied to the Pilatus PC-12 or 21 model aircraft shown in FIG. 2, using the second method the distance B to the throat is approximately 2.5 inches. Using a C/B ratio of 0.65, the reduced thickness C in the region of the exhaust flow transition connection would also be approximately 1.63. Using the preferred 40% ratio for F/C and a calculated inlet section reduced thickness C of 1.63, the reduced thickness of the transition section is 1.63*0.4=0.65. The exhaust supply tube connection diameter used for the Pilatus PC-12 is 1.38, thus the thickness reduction ratio E/F for the connection transitions 9 a and 9 b is 1.38/0.65=2.12, which compares with the greater than 2 ratio typical of the thicker section inlet 5 shown in FIG. 3A. The flow area of the 1.38 diameter tube exhaust supply tube is approximately 1.5 in² (ignoring the tube wall thickness), thus the width (G and G′) of the transition connections 9 a and 9 b would need to be approximately 1.5/.65=2.3 inches to provide equivalent flow area.

It should be noted that the temperature of the inlet can be varied by the amount of exhaust gases passing thru the inlet cavity 7, which, in turn is controlled by the diameter of the exhaust tubing connected to the inlet as well as the channel flow area of the transition connections 9 a and 9 b. Alternatively, a valve 41 (FIG. 4) could be inserted in tubes 11 a or 11 b to adjust the exhaust flow into or exiting the inlet cavity 7. This valve could be, e.g., a simple butterfly valve that can be, e.g., either thermostatically or manually controlled.

In practice, due to the relatively short length of the transition connections 9 a and 9 b and the high exhaust temperatures, it has been found that a reduction in flow area in the connection transitions of up to 20% can be tolerated without significant impact of the inlet's anti-ice performance. Referring to FIGS. 5 and 9, e.g., for a single engine aircraft, such as the Pilatus PC-12 and 21 aircraft, the exhaust gases flow into the inlet lip 4 from the engine's left exhaust 14 a, through transition connections 9 a, then out transition connections 9 b and into the engine's right exhaust 14 b. Obviously a counter flow arrangement would provide a similar function. Also, because the inlet 4 is rigidly attached to the airframe cowling, the prior art has recognized that flexible connections 12 a and 12 b need to be placed between the connections at the engine's exhaust stacks 14 a and 14 b and the connections to inlets 4 and 5 (reference FIGS. 4 and 5). To stabilize the exhaust tube connections 11 a and 11 b to the transition connections 9 a and 9 b, FIGS. 5, 14A and 14B show a support bracket 22 connected to engagement bracket 34 which provide for the support of the exhaust tubes 11 a and 11 b at the transition connections 9 a and 9 b. The support bracket 22 is attached to either or both the outer surfaces 8 b of the duct 8 or inner surface 3 a of the cowling 3.

FIG. 6 shows some additional details of the invention. Hot exhaust gases enter the transition connection 9 a and flow into a cavity 7 within the ram-air inlet lip 4 formed by the bulkhead 4 a attached to the inlet 4. A partition is inserted in the cavity 7 between the inlet and outlet transition connections, forcing the hot gases to circulate through the inlet cavity 7, around the inlet 4, and out the transition connection 9 b.

Due to the intense heat of the exhaust gases, this invention provides additional means to protect the internal surfaces of the cowling 3, as well as the external surface 8 a of the engine intake duct 8. As shown in FIGS. 6 and 10, external air flow entering the lower portion of the cowling, e.g., through inlet opening 20, is channeled through ducts 16 a and 16 b to provide cooling air flow between the connection transitions 9 a and 9 b and adjacent cowling 4 and duct 8 surfaces. In the preferred arrangement shown in FIG. 10, one of the external air flow cooling ducts 16 a directs most of its cooling air flow out its exit 22 a and onto the cowling 3 surface, and the other duct 16 b directs most of its cooling air flow out its exit 22 b and onto the engine intake duct 8 surface. Obviously, the arrangement of which duct cools which surface could be reversed or both ducts could be used to cool both surfaces. FIG. 11A shows a few NACA style flush inlets 17 a,b and 20, used to supply the cooling air placed on the lower surface of the cowling. In practice, one or more flush inlets or inlets of alternate design could be used and placed at alternate locations on the cowling to feed air to one or more ducts channeling cooling air as required.

An alternative cooling airflow arrangement is shown in FIGS. 12 and 13. In this configuration, a plenum chamber 21 is formed aft of the external air inlet 20 by insertion of a bulkhead partition 18. One or more external air inlets in the cowling provide airflow into the plenum chamber. The cooling air flow entering the plenum 21 exits through an opening 18 a in the bulkhead partition 18 located in the region of the transition connections 9 a and 9 b thereby providing cooling airflow to the adjacent cowling 3 and duct 8 surfaces. Also shown in FIG. 12 is the application of thermal protection materials 25 to the adjacent cowling 3 and duct 8 surfaces. In addition, as shown in FIG. 7, a thermal barrier coating 27 may be applied directly to the surfaces of the transition connections 9 a and 9 b. Also, an external heat shield 11 placed around the transition connections 9 a and 9 b, such as that shown in FIG. 8, may be provided.

FIGS. 14A and 14B show the exhaust tube coupling connection means utilized for the invention. As shown in FIG. 5, the inlet duct lip 4 is attached to the cowling 3 and the support bracket 22 is attached to the upper surface of the intake duct 8. Referring to FIG. 14A, an engagement bracket 34 is rigidly attached to the support bracket 22. Sleeves 36 are attached to the ends of the exhaust tubes 11 a and 11 b and are inserted into the engagement cylinders 37 that are rigidly attached to the engagement bracket 34. As shown in FIG. 14B, once fully inserted, the sleeves 36 also insert over (or alternatively into) the round ends of the transition connections 9 a and 9 b. The sleeves 36 contain one or more protruding keys 39 attached to, or integrated with, the sleeve 36 that are inserted into and slide through slots 38 in the engagement cylinders 37 when the axis of the long straight section of the exhaust tubes 11 a and 11 b are placed approximately in the vertical position. Once fully inserted, the exhaust tubes are rotated (exhaust tube 11 a is rotated counter clock wise looking forward, exhaust tube 11 b is rotated clock wise looking forward) so that the sleeve keys 39 no longer align with the engagement cylinders slots 37 and 38, thus preventing the exhaust tube sleeves 36 from disengaging the coupling connection with the transition connection 9 a and 9 b. Brackets 40 a and 40 b secure the exhaust tubes rigidly to the cowling thereby preventing the exhaust tubes from rotating and subsequent separation of the coupling.

It should be noted that the turboprop compact inlet geometry described herein and illustrated in FIGS. 11A and 11B will also provide excellent ram-air recovery performance for inlets that do not incorporate engine exhaust flow heating deice (or anti-ice) means or alternate deice means such as electric heat element boots, or application of deicing liquid (TKS Systems).

Also, although the turboprop compact inlet geometry described herein is focused on aircraft tractor configurations wherein the propeller is positioned in front the inlet, the inlet geometry described herein is also applicable to aircraft pusher configurations wherein the propeller is located aft of the inlet.

The examples and descriptions given above illustrate possible embodiments of the present invention. While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments but should be defined only in accordance with the following claims and their equivalents. 

The following invention is claimed:
 1. An engine for a turboprop powered aircraft incorporating an engine ram air intake duct, the ram-air intake duct comprising a ram-air inlet lip defining the opening into the engine ram-air intake duct, the ram-air inlet lip presenting a closed, ovoidal, forward facing shape; a spinner and propeller blades secured to the spinner, the spinner being rotatively connected to the engine cowling forward of the cowling and ram-air inlet lip; wherein the engine ram-air inlet lip extends rearwardly from a position rearward of the boundary of the spinner and cowling; a reference line extending parallel to the centerline of the spinner, rearwardly and forwardly from the forwardmost point of the ram-air inlet lip, extends at a distance below the centerline of the spinner and divides the interior of the ram-air inlet lip such that the portion within the inlet lip above the reference line parallel to the centerline of the spinner is smaller than the portion within the inlet lip below that reference line.
 2. The engine for a turboprop powered aircraft incorporating an engine ram air intake duct of claim 1, wherein the maximum thickness or height of the portion above the reference line is less than 40% of the total thickness of the ram-air inlet lip.
 3. The engine for a turboprop powered aircraft incorporating an engine ram air intake duct of claim 2 wherein the maximum thickness or height of the portion above the reference line at the duct throat, is greater than 25%.
 4. The engine for a turboprop powered aircraft incorporating an engine ram air intake duct of claim 2 wherein the maximum thickness or height of the portion above the reference line at the duct throat, is between 25% and 35% of the inlet intake air opening height (opening vertical dimension) measured at the location of the inlet duct throat.
 5. The engine for a turboprop powered aircraft incorporating an engine ram air intake duct of claim 1 wherein the maximum thickness or height of the section measured at the inlet throat is between 60% and 75% of the distance from the inlet lip face to the location of the inlet throat.
 6. An engine cowling for a turboprop aircraft incorporating a ram-air intake duct comprising a ram-air intake duct inlet lip having a closed, approximately oval profile relative to the front view of the engine nacelle, where in the inlet incorporates means to deice the inlet lip by passing a portion of the engine exhaust flow into the interior of the inlet lip and wherein the exhaust flow connections to the interior of the inlet lip flow through a circular to non-circular, or ovoidal, transition connection attached to the inlet lip interior, wherein the vertical thickness of the transition connection is reduced to 50% or less than the diameter of the of the exhaust round tube connection.
 7. The engine cowling for a turboprop aircraft incorporating a ram-air intake duct of claim 6 wherein the exhaust flow connections to the inlet transition connection are supported by a bracket attached to the outer surface of the engine air intake duct.
 8. The engine cowling for a turboprop aircraft incorporating a ram-air intake duct of claim 6 wherein the transition connection has a minimal flow area between 80% and 100% of the flow area of the exhaust connection round tube.
 9. An engine cowling for a turboprop aircraft incorporating a ram-air intake duct lip in accordance with claim 10, wherein the surfaces of the cowling and intake duct in close proximity to the exhaust flow connections to the inlet lip are cooled by air flow from a plenum chamber within the cowling receiving cooling air from openings in the cowling.
 10. An inlet for the Pilatus PC-12 and 21 models incorporating the design elements specified in claim
 11. 11. An inlet for the Pilatus PC-12 and 21 models incorporating the design elements specified in claim
 12. 12. The engine cowlings for a multi-engine turboprop aircraft, each engine incorporating a ram-air intake duct of claim
 1. 13. The engine cowlings for a multi-engine turboprop aircraft, each engine incorporating a ram-air intake duct comprising a ram-air intake duct inlet lip having a closed, approximately oval profile relative to the front view of the engine nacelle in accordance with claim
 6. 